getElementByID -> It will show run-time error as
"Object doesn't support property or method 'getElementByID'"

ClientID -> If we provide ClientId or clientId then it will give compile-time error as

"System.Web.UI.WebControls.TextBox' does not contain a definition for 'ClientId' and no extension method 'ClientId' accepting a first argument of type 'System.Web.UI.WebControls.TextBox' could be found (are you missing a using directive or an assembly reference?)"